Recent and developing applications for compliant hydrodynamic gas bearings require operation at high rotational speeds, extremely high temperatures, under heavy loads and in the absence of oil lubrication. The gas turbine is a prime example of this application. Uses of the gas turbine are expanding rapidly and include prime mover and auxiliary power systems for aviation, marine and automobile applications. The compliant hydrodynamic gas bearing offers potentially higher operating temperatures, elimination of oil lubrication requirements and limitations, greater accomodation of thermal distortion, assembly variations, tolerance of dynamic shaft motion because of bearing compliance, reduced frictional power loss, reduced rotor noise and lower bearing costs. Accordingly, this bearing is ideal for gas turbine applications.
One troublesome problem that has delayed full implementation of the compliant hydrodynamic gas bearing has been damage to the bearing surface during start and stop phases of operation. The supporting gas film generated in a hydrodynamic bearing due to relative rotation of the bearing surfaces is not sufficient to support the rotor until it reaches a certain speed. At that time, the gas film lifts the rotor from the bearing surface and thereafter the gas lubrication prevents further contact between bearing surfaces. Although the relative rotation during the contact phase is quite slow, the cumulative effect of the contact can be sufficient to gall the bearing surfaces. In addition, shocks or violent eccentric loading of the rotor can cause momentary contact of the bearing surfaces which also can result in galling of the bearing surfaces. The galling can weaken the flexible bearing sheet and can actually interfere with the operation of the bearing during the hydrodynamic phase of the operation.
One solution which has been attempted in the past is to provide a low friction coating on the bearing surfaces. Low friction coatings appear to be promising, but are not usually adaptable to application in compliant bearings because of the particular requirements. In general, the known prior art coatings have been developed for rigid members, whereas, in compliant bearings, the coating is applied to a flexible bearing sheet which is continually flexed during operation. As a consequence, the coating itself must be flexible or it will crack and possibly break up during operation. If this happens, the bearing surface can become damaged very quickly and, even worse, the abrasive wear products of the crumbling coating can themselves greatly shorten the life of the bearing. To achieve flexibility, the coating must be applied in a very thin layer, but a very thin layer will wear through rapidly if it is not extremely tough. In addition, the coating material must bond permanently to the substrate and be unaffected by flexing, temperature changes and harsh environment agents. Finally, the coating must perform well at ambient or start-up temperature, at the normal operating temperature of the machine on which it is installed, such as a gas turbine, and at all intervening temperatures. For example, when the engine is started, it can be quite cold. When it is stopped, the temperature of the bearing is normally at the operating temperature of the engine which can be in the neighborhood of 430.degree. C. Coatings which function well at lower temperatures are often found to deteriorate at higher temperatures. Some high temperature coatings which function well at elevated temperatures do not provide good anti-friction characteristics at lower temperature.